Theft of conductive materials, such as metal, has long been a problem due to the high monetary value of the materials. For instance, the high scrap value of copper and aluminium, which are commonly used in the manufacture of electrical cables, water pipes and the like, makes the cables, pipes etc. an attractive target for theft. Such theft can be both expensive to repair and extremely disruptive to people who rely on the infrastructure of which the cables, pipes etc are part. Metal theft commonly occurs in respect of rail systems and electricity networks and, in both cases, is very disruptive. Metal theft can occur if a network is energised or de-energised. Thieves stealing from an energised network may first isolate the network or may instead sever the network (thereby to de-energise it) prior to removal of the metal.